Hi Rob,
You should be able to startup off of the OS X installation disks that came with your Mac. Choose the “archive and install” option to replace your newer OS with the original one (be sure to check the options to save your files, etc). Once installed you can then update to whatever OS version seems to be working for you (find the various versions on Apple’s website). I would however, check with your capture card manufacturer for any updates or any workarounds for the problems you are having with OS 10.4.5. It could be your plugins, drivers, firmware, etc aren’t up-to-date and that would be a much simpler fix than reinstalling an older version of OS X.
Tony!